This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Attention is a critical factor in sensory and cognitive performance. Problems in controlling and directing attention can lead to serious deficits in performance and behavior. More complete information about the neural mechanisms underlying attention will be important for understanding how it facilitates perception and guides behaviors. Our experiments will extend our understanding of the neuronal basis of attention by testing the hypothesis that attentional modulation depends on the same neuronal circuits that mediate sensory response normalization. Response normalization is a well-established mechanism that adjusts sensory signals based on the overall activity in a large group of neurons with similar response properties. We propose that attentional modulations in visual cerebral cortex act through this normalization mechanism, such that attentional modulation cannot occur in conditions when normalization is weak or eliminated. These experiments will provide a critical test of the hypothesis that attention depends on normalization mechanisms. In doing so they have the potential of greatly extending our understanding of the neural mechanisms that underlie attention.